The invention relates to a method for digital multi-channel data transmission with a COFDM method which operates in a frame oriented manner with the frame containing at least one zero symbol.
The method according to the invention is used in the COFDM (coded orthogonal frequency division multiplex) method provided in digital radio, for example, it may be used in connection with transmitter identification in the common wave frequency network.
A common wave frequency network is characterized in that a plurality of transmitters radiate the same signal simultaneously. The COFDM modulation method is highly immune against multipath distortions and is thus basically suitable for common wave networks.
The digital radio signal should, among other things, contain the identification of the transmitter, something which allows the receiver, for example, to determine its approximate position and to subsequently use this information, for example, in the selection of interesting traffic information. The identification of the transmitter, however, can only occur by way of the signal radiated from it. This is a problem in common-wave networks, because all transmitters in the network radiate the same multiplex signal. Signalization of the transmitter identification must therefore take place outside of the actual multiplex signal.
COFDM is a digital multi-channel modulation method. In this type of method, the data signal to be transmitted is divided into a number N (for example, several hundred) of sub-channels, which in the frequency domain lie closely adjacent to one another, with the spectra definitely overlapping one another. Due to this division, the data rate transmitted in each subchannel is only a fraction of the original data rate. The duration of the symbol increases at the same ratio or proportion. This is advantageous, if the echoes occur on the transmission path. If a sufficiently great N is selected, it is possible to obtain a duration of the symbol which is long compared with the maximal echo delay. This results in the symbol interference caused by echoes to be reduced such that the signal can be demodulated without distortion.
The COFDM transmission signal s(t) may be represented in the base band as a superposition of time and frequency-shifted base pulses b(t): ##EQU1##
In this case the base pulse is defined by: ##EQU2## j=.check mark.-1 is the imaginary unit.
The summation index i represents the symbol clock pulse, and index k the subchannel. The following parameters describe the COFDM modulation method:
T.sub.s : Symbol duration PA1 t.sub.s : used symbol duration PA1 t.sub.s : protection time PA1 F.sub.s : subchannel spacing PA1 The first symbol block s.sub.1 in the frame is the symbol zero. It is characterized in that no transmission signal is radiated. The receiver can detect the drop in field strength by means of an envelope detector. The distance between the zero symbols of the nth and the (n+1)th frame serves frame synchronization while the duration of the zero symbol serves for symbol synchronization. The zero symbol is not processed by the FFT. As a rule, its length T.sub.o =0.8 T.sub.s, deviates from T.sub.s. PA1 The second symbol block s.sub.2 in the frame is the phase reference symbol. It is needed to initialize the differential demodulator, among other things. PA1 The remaining frame comprises symbol blocks carrying information.
They are related by way of the relationships T.sub.s =t.sub.s +t.sub.g and F.sub.s =1/t.sub.s. As a further characteristic parameter, the useful component, the quotient of used symbol duration and symbol duration .gamma.=t.sub.s /T.sub.s can be defined. The variants of COFDM currently discussed for use in digital radio is always .gamma.=0.8. Taking these relationships into consideration, only one free parameter remains in the COFDM system and the remaining parameters may be derived from it.
The data to be transferred are coded in complex symbols d.sub.i,k. In COFDM the 4-phase shift is used as the modulation method, and therefore d.sub.i,k {1, j -1, -j} is true. In order for the transmission not to be sensitive to channel-dependent phase rotations, transmission symbols s.sub.i,k which are generated by differential coding s.sub.i,k =s.sub.i-1,k d.sub.i,k are radiated rather than d.sub.i,k. It might be practical to combine the transmission symbols of all subchannels transmitted in the same time slot i into an N-dimensional vector s.sub.i. Such a vector is mentioned in the symbol block.
The generation of the COFDM transmission signal is performed, for example, digitally, with the help of the inverse fast Fourier Transformation (IFFT). The block circuit diagram of a COFDM transmitter is shown in FIG. 1a. For each time slot i an IFFT is calculated, whose output signal has the duration t.sub.s. It is continued periodically on a signal of the duration T.sub.s.
The COFDM demodulator serves to recover the symbols d.sub.i,k carrying the information. To accomplish this the values ##EQU3## are generated from the receiver input signal r(t). From these values estimated values d.sub.i,k =r.sup.*.sub.i,k r.sub.i-1,k are obtained for the data symbols by way of differential demodulation d.sub.i,k =r.sup.*.sub.i,k r.sub.i-1,k. In this case, r.sup.* is conjugated complex to r.
The COFDM demodulator may also be realized digitally with the help of the fast Fourier transformation (FFT). It is shown in FIG. 1b. For each time slot a segment having the duration t.sub.s of the receiver signal is evaluated. A segment of the duration t.sub.g remains unevaluated. The echoes of the signal from the preceding time slot fall into this slot.
The data transmission in the COFDM system is frame oriented. A frame is a structural arrangement of symbol blocks in temporal succession. It has the following structure, which is illustrated in FIG. 2:
It is the object of the invention to provide a method of this type for the digital transmission of data and which allows for digital data to be transmitted independently outside the actual-digital multiplex system. This object generally is attained according to the present invention by a method for digital multi-channel transmission particularly, a COFDM method which operates in a fame oriented manner with the frame containing at least one zero symbol, data additionally are transmitted in at least one zero symbol. Advantageous configurations and/or modifications likewise are disclosed.